The invention relates to a method of detecting an interfering signal in a time division multiple access (TDMA) radio receiver.
When information is transferred over a radio channel, a signal to be transmitted has to be modulated. In modulation, the aim is to obtain a signal of a form which can be transmitted at a radio frequency. The prerequisite of a good modulation method is, for example, that it enables maximum amount of information to be transferred in a narrowest possible frequency band. Depending on the purpose of use, other features may also be stressed. Furthermore, modulation must not generate more than a minimum amount of interference to a neighbouring channel.
One of the modulation methods is called π/4-DQPSK (π/4-shifted, Differential Quaternary Phase Shift Keying) modulation. This modulation method comprises eight phase states, but only four phase shifts. The accepted phase shifts (symbols) are ±π/4 and ±3π/4. FIG. 3A shows a modulation constellation diagram. Each phase shift corresponds to two bits that are to be transmitted. In other words, a digital signal modulates a carrier in two-bit sequences in such a way that a given phase shift corresponds to each two-bit combination during each symbol sequence. A symbol sequence refers here to a signal sequence which is used for transmitting the two bits. The phase shifts, which correspond to bit combinations 00, 01, 10 and 11, are π/4, 3π/4, −π/4 and −3π/4. The symbol frequency used by the Terrestrial Trunked Radio system (TETRA), for example, is 18 kHz and, consequently, the bit frequency is 36 kHz.
When a signal is received it has to be demodulated, i.e. the bits which are modulated to the signal have to be detected by a detector so as to find out the information included therein. In transmission over a radio path, an interfering signal may sometimes appear at the frequency used, which interfering signal uses a different modulation method from the one used by the actual signal. In the TETRA system, for example, such an interfering signal can be detected by examining the training sequence, which is always included in a timeslot, of the received timeslot. When the bit error ratio of the training sequence is calculated by comparing the training sequence with the stored version of the training sequence and the bit error ratio exceeds a given threshold value, then the received signal is found out to be an interfering signal.
A problem in the above arrangement is that factors, such as strong signal fading, which degrade the quality of reception may occur during the training sequence of the actual signal. The number of bit errors may then become so large that an interfering signal is mistakenly thought to exist.